The present invention relates to hydrogenation catalysts for hydrocarbon oils and processes for hydrogenation and more particularly to a hydrogenation catalyst which is effective in hydrodesulfurization of petroleum hydrocarbons containing a gas oil fraction or a kerosene fraction and a process for hydrogenation using the catalyst.
In recent years, awareness of environmental issues and air pollution has been raised, and particularly, has been directed to sulfur compounds contained in fuels used for transportation applications. For example, gasoline engines have been strongly demanded to be improved in fuel efficiency not only in the sense of resource conservation or economical factors but also in the sense of a reduction in carbon dioxide emissions. Therefore, the development and promotion of new combustion systems such as lean burn engines and direct gasoline-injection engines have been progressed under these situations. However, the components constituting the exhaust gas discharged from these engines are not always the same as those to be treated with the conventional ternary exhaust gas treating catalysts, on which further improvements have been required. It is indicated that the sulfur compounds contained in gasoline adversely affect such newly developed exhaust gas treatment systems or catalysts. Currently, the naphtha fraction produced from a fluid catalytic cracking (FCC) unit processing mainly a vacuum gas oil fraction as a feedstock has been used as a base gasoline at a fair percentage. In order to decrease the sulfur content in gasoline pool, it is necessary to decrease the sulfur content of the gasoline fraction produced in FCC. It is, therefore, necessary to decrease the sulfur content of a final gasoline by any of or combination of decreasing of the sulfur content of the vacuum gas oil, desulfurizing in an FCC apparatus, and decreasing of the sulfur content of the FCC gasoline.
On the other hand, in addition to chemical substances such as SOx and NOx, fine particles so-called “particulates” are contained in the exhaust gas discharged from a diesel engines using gas oil and are in danger of harming the human health. It is proposed to mount a particulate trap filter such as DPF or a system capable of burning particulates downstream of an engine in order to remove the particulates. The use of such devices in diesel powered automobiles have also been studied. Furthermore, reduction catalysts for removing NOx are developed. However, these devices and catalysts are likely to be poisoned or deteriorated with SOx produced due to combustion of sulfur compounds in fuel. Such deterioration of the exhaust gas purification system or catalyst is a serious problem for diesel powered automobiles such as trucks that run longer distance than gasoline-fueled automobiles. In order to solve this problem, it is strongly demanded to decrease the sulfur content in gas oil as much as possible. Gas oil used as a diesel fuel contains a fraction classified as a kerosene fraction by its boiling range in such a certain percentage that the product properties are optimally maintained. Therefore, it is necessary to decrease the sulfur content both in the kerosene fraction and the gas oil fraction so as to decrease the total sulfur content in gas oil used as fuel. Furthermore, it has been required to decrease the aromatic content in fuel oil, regarded as substances responsible for causing the generation of particulates. A kerosene fraction used as fuel for various heating devices such as stoves can be decreased in the amount of sulfur oxide or the like which is harmful, by decreasing the sulfur content. The generation of such harmful substances gives directly significant influences to the human body because most of these heating devices are used indoor.
The kerosene fraction, gas oil fraction and vacuum gas oil fraction produced by distilling crude oil or cracking fuel oil generally contain 0.1 to 3 percent by mass of sulfur compounds and thus are usually used as a base gas oil after being hydrodesulfurized. The main sulfur compounds contained in these fractions are thiophene, benzothiophene, dibenzothiophene, and derivatives thereof. Each of these fractions contains a certain amount of sulfur compounds with a relatively poor reactivity in its heavy portion. For example, the gas oil fraction contains alkyl-substituted dibenzothiophenes having a plurality of alkyl groups as substituents, such as 4,6-dimethylbenzothiophene that are poor in reactivity and inhibit the desulfurization of the fractions from proceeding to a low sulfur level of 10 ppm by mass. It is presumable that the activating function of a catalyst required for removal of such sulfur compounds will be different from that of a catalyst with the conventional activation range.
Furthermore, the gas oil fraction contains several hundreds ppm by mass of nitrogen compounds. Such nitrogen compounds are known as substances inhibiting hydrodesulfurization because they are adsorbed to the desulfurization active sites on a catalyst competitively with sulfur compounds. It is generally pointed out that denitrogenation reaction proceeds associated with hydrogenation of aromatic hydrocarbons, and thus the higher hydrogenation activity the more the catalyst is advantageous.
Hydrogenation catalysts commonly used for petroleum refining comprise cobalt, or nickel and molybdenum, supported on an inorganic porous support. In order to further improve the desulfurization activity of such catalysts, various inorganic porous supports have been studied in terms of their materials and physical properties. Alumina is typically exemplified as an inorganic porous support. However, other than alumina, Japanese Patent Laid-Open Publication No. 7-197039 proposes a support comprising zeolite that is crystalline and has a large surface area and acidic properties, and Japanese Patent Laid-Open Publication No. 6-127931 proposes a support containing silica. Further, Japanese Patent Laid-Open Publication No. 8-28117 proposes a support containing silica and supporting phosphorus together with active metals.
It is presumable that a support containing alumina and silica has a large surface area and forms a slight number of acid sites. There is a possibility that these acid sites facilitate the desulfurization activity by giving some influences on the desulfurization active sites. On the other hand, it is generally known that phosphorus acts together with active metals and gives influences on the dispersion and active site structure formation of the active metals. However, the mechanism of the action is so complicated that the physical properties of alumina, silica, and phosphorus suitable for forming a desulfurization active site exhibiting a high activity have not been explicit well. The physical properties of these components for enhancing the denitrogenation activity has not become apparent yet as well. The conventional desulfurization catalysts are not sufficient in activity to achieve an extremely high depth of hydrogenation at which a fuel is decreased in sulfur content to 10 ppm by mass or less and in nitrogen content to 3 ppm by mass or less, respectively. It is, therefore, necessary to control the physical properties of the catalyst more specifically and more precisely.